1. Field of the Invention
This invention relates to microporous titanias and zirconias and a method of making these materials.
2. Description of the Previously Published Art
Many liquid chromatographic packings are formed of silica, either alone or with some coating or surface treatment. Silica has good physical strength, and may be formed into a variety of particle shapes and sizes with a variety of pore diameters, porosities, and surface areas. However, silica has the disadvantage that it is readily soluble in basic media. This flaw precludes the use of silica-based packings in applications where the chromatography itself or some customary washing of the chromatographic material requires the use of a base.
Several materials have superior base-resistance to silica. Titania, or titanium dioxide, is a superb choice from among these, being insoluble in most acidic or basic solutions, as well as being non-toxic and not being very expensive. Although titania has been described as a liquid chromatographic support, it suffers in this regard because it is difficult to produce porous titania bodies which have the combination of physical integrity and sufficient porosity and surface area for use in liquid chromatography.
Physical integrity is a concern with respect to chromatographic media for two reasons. First, high flow rates in high-performance liquid chromatography (HPLC) columns result in high backpressures, which may reach 10,000 psi in some cases. Useful supports must resist deformation caused either directly by liquid flow or by the backpressure. Second, in order to assure good stability of the packed column under flow conditions, and to keep void volume low, vigorous column packing methods are used. For example, the slurry packing method, as set forth by Pfannkoch, Switzer, and Kopaciewicz (Journal of Chromatography, 1990, 503, 385-401), requires that the packing be forced into the column as a slurry with pressure bursts of about 5,000 psi. During the packing procedure, both attrition and crushing of packing particles may occur. Thus, these particles must have both physical strength and attrition resistance.
Porous titanias may be produced through the agglomeration of smaller, non-porous particles ("ultimate particles"). When the ultimate particles are agglomerated, the spaces between them become pores, and their surfaces provide surface area to the final body. Some of the surface area and pore volume will be lost where the ultimate particles are fused. The practical difficulty in the agglomeration process is providing sufficient attachment between the ultimate particles to impart the desired strength to the porous body, while maintaining suitable pore volume and surface area. Strong, but non-porous, and porous, but weak, titania bodies are easily formed.
U.S. Pat. Nos. 3,855,172 and 4,010,242 disclose porous microspheres of refractory oxides, including titania, and a method for their preparation. Colloidal particles of the oxide are trapped in a polymerizing mixture of formaldehyde and urea or melamine. Upon polymerization, microspheres are formed, from which the polymers may be removed by calcination to produce pores. However, according to Example 10 of both patents, when this technique is applied to titania, only solid non-porous particles with surface area less than one square meter per gram are formed. U.S. Pat. No. 3,782,075 also discloses porous oxide microspheres for use in chromatography, but refers to the above-mentioned method of U.S. Pat. No. 3,855,172 for their manufacture.
U.S. Pat. No. 4,245,005 teaches the use of porous oxide supports with polymeric coatings in liquid chromatography. The claims in this patent are directed to the polymeric coating, which is formed from polyethyleneimine cross-linked with diepoxides. Although titania supports are discussed, no mention is made of their preparation. Similar information is disclosed in an article entitled "Preparation and Evaluation of Inorganic Anion Exchange Sorbents Not Based on Silica", published in the Journal of Chromatography, volume 359, pages 121-130, in 1986. However, no reference to the method of preparation of suitable porous titania is given, and the titania mentioned is of a particle size too large to be suitable for HPLC.
U.S. Pat. No. 3,892,580 discloses inorganic bodies which have a high surface area due to inherent high porosity. These materials are useful as catalyst supports, filter media and as carriers in chromatographic columns. The primary object in this patent is to provide a method for making porous bodies of alumina, titania, zirconia and silica. The pore diameters of these bodies will range from about 100 Angstroms to 1,000 Angstroms. These porous bodies are made by mixing a liquid binder solution with a colloidal suspension of the inorganic oxide to yield a mixture of uniform consistency. This mixture is then dried and fired at a temperature below the sintering temperature of the inorganic oxide. It is disclosed in column 3 of this reference that the liquid binder that is employed should not dissolve or substantially erode the inorganic oxide particles since they will then no longer permit the achievement of a controlled pore dimension. This is the function of the initial particle size of the particles. In this regard it is stated that this essentially eliminates the common mineral acids such as hydrofluoric acid, hydrochloric acids, sulfuric acid, nitric acid and the like from consideration. However, it was found that certain organic acids and organic acid salts had the utility to be good binders. These acids and acid salts include acetic acid, propionic acid, sodium acetate, magnesium acetate, and zinc acetate. The method of producing the inorganic oxide bodies consist of forming a slurry of the particles and the binder and then drying this slurry at a temperature of about 90.degree. C. to 100.degree. C. During the drying step the bodies of the inorganic material are formed. These bodies are then fired at an elevated temperature to develop the chemical bonds. In the process of this patent the strength of the body and of the formation of the oxide bonds are the result of the firing step rather than one of the prior steps of the process.
3. Objects of the Invention
It is an object of this invention to obtain porous titania and zirconia microspheres.
It is further object of this invention to obtain porous monodisperse titania and zirconia microspheres.
It is further object of this invention to obtain porous monodisperse titania and zirconia microspheres with both good porosity and physical strength.
It is further object of this invention to produce porous titania and zirconia supports which are suitable for use in high-performance liquid chromatography.
It is further object of this invention to provide titania supported catalysts.
It is further object of this invention to provide a zirconia supported catalysts.
These and further objects will become apparent as the description of the invention proceeds.